Because, say the plaintiffs' lawyers, it shouldn't have let PCs that could only run Vista Home Basic, the runt of the litter of Window Vista versions, describe themselves as "Vista Capable". By doing so, claim the lawyers, Microsoft "unjustly enriched itself".

Now Marsha Pechman, a federal judge, has determined that all the individuals who had this complaint can band together to create a single complaint against Microsoft because, as she put it, "common issues predominate". She elucidated: "These common issues ... are whether Vista Home Basic, in truth, can fairly be called 'Vista' and whether Microsoft's 'Windows Vista Capable' marketing campaign inflated demand market-wide for 'Windows Vista Capable' PCs."

Clearly the outcome will be judged on those two questions. So let's see if we can work them out, shall we?

First, is Vista Home Basic truly Vista? The comparison chart, handily provided by Microsoft at tinyurl.com/3843r3, shows Home Basic against its siblings - Home Premium, Business and Ultimate. It ticks just 3 of 17 boxes. (Home Premium, 13; Business, 11; Ultimate, all 17.) There's none of the collaboration features all its siblings share, nor mobility centre, nor Aero desktop, nor secondary display drivers, nor scheduled backups. Honestly, it's starting to sound more like Windows ME, actually.

Next, did calling PCs capable only of running this three-legged Vista "Vista Capable" inflate demand market-wide for such PCs? If it did, then it did a fairly rubbish job of it: while 250m PCs were sold last year, best estimates suggest only 100m Vista licences were (there's no breakdown by category, though).

The worst thing? Internal Microsoft emails acquired by the plaintiffs suggest unease within the company about the campaign - including Windows chief Jim Allchin writing "you guys have to do a better job with our customers". Umm, think you may be too late on that one, Jim.